The 3.5/Pathfinder Handbook

[Starbuck_II]

[*]Under Pathfinder Rules: Bob has Strength 18, a MW weapon, and +2 BAB (humans in Pathfinder get +2 to one stat, ie Strength for a Fighter). He makes one roll with his Combat Maneuver Bonus of +8 (+4 for Strength, +2 BAB, +2 Improved Trip) against the Ogre's CMD of 17 (10 + 3 BAB + 5 Str -1 Dex +1 Size). 60% chance of success, and you aren't counter-tripped unless you fail by 10 or more (which is impossible in this case). So the odds are more favourable, but the payoff is lower (since Improved Trip doesn’t grant a free follow-up attack anymore).

I'm pretty sure +8 vs 17 is 55% chance.
Because if you needed a 10 on a 1d20 that would be about 50% chance, but you need a 9 thus 55% chance.
Why the extra 5% (60-55 =5)?

But good handbook either way.